Surface deformation imaging process

ABSTRACT

AN IMPROVED IMAGING PROCESS FOR ELECTRON BEAM RECORDING IS DISCLOSED. IN THIS PROCESS, A LAYER OF A HIGHLY VISCOUS PHOTOPOLYMER IS EXPOSED TO AN ELECTRON BEAM IN A VACUUM. IT HAS BEEN FOUND THAT THE ELECTRON BEAM SIMULTANEOUSLY CAUSES THE FORMATION OF A SURFACE DEFORMATION IMAGE WHILE POLYMERIZING THE MATERIAL IN EXPOSED AREAS. THUS, AN INSTANTANEOUS VISIBLE IMAGE IS FORMED WITH NO POST PROCESSING OF THE MATERIAL REQUIRED. BACKGROUND AREAS OF THE MATERIAL MAY BE POLYMERIZED BY EXPOSURE TO SUITABLE RADIATION, SUCH AS ULTRA VIOLET LIGHT, IN THE SUBSTANTIALY ABSENCE OF OXYGEN. THIS PRODUCES A STRONG, EASILY HANDLED IMAGED SHEET.

Patented July 4, 1.972

3,674,591 SURFACE DEFORMATION IMAGING PROCESS Sherman H. Boyd, Jr., La Mesa, Calif., assignor to Stromberg DatagraphiX, Inc., San Diego, Calif. No Drawing. Filed Nov. 28, 1969, Ser. No. 880,970 Int. Cl. G03c 9/08 U.S. Cl. 156-272 14 Claims ABSTRACT OF THE DISCLOSURE An improved imaging process for electron beam recording is disclosed. In this process, a layer of a highly viscous photopolymer is exposed to an electron beam in a vacuum. It has been found that the electron beam simultaneously causes the formation of a surface deformation image while polymerizing the material in exposed areas. Thus, an instantaneous visible image is formed with no post processing of the material required. Background areas of the material may be polymerized by exposure to suitable radiation, such as ultra violet light, in the substantial absence of oxygen. This produces a strong, easily handled imaged sheet.

BACKGROUND OF THE INVENTION Imaging systems in which the surface of a deformable dielectric material is deformed in image configura tion are now well-known. Typical of these systems are the frost system described by Gunther et al., in U.S. Pat. No. 3,196,011, and the relief system described by Glenn in US. Pat. No. 2,943,147. In each of these systems, a latent image is first produced by forming an electrostatic charge pattern on the surface of a thermoplastic layer. Then the thermoplastic is heated to its softening temperature at which point surface deformation occurs in image areas. In frost imaging the deformation occurs across uniformly charged areas whereas in relief imaging the deformation appears at the location of electrostatic charge gradients and not uniformly across uniformly charged areas. In either case, the image may be erased by heating the material above its softening temperature in the absence of electrostatic charge fields. In developing the images, the heating step is often critical since the temperature of the material must be raised to a point at which it will deform, but not to a point at which the electrostatic charge dissipates and the entire surface becomes so soft that the image is erased. Many materials have the characteristic of increasing conductivity as the temperature increases. With these materials, ex cessive heating may permit the charge pattern to dissipate before the surface deformation image is formed.

From time to time, it may be desirable to add further information to a previously imaged layer. It is difficult to do so with the known surface deformation systems, since heating one area on a sheet to develop as newlyadded electrostatic image is likely to heat adjacent previously imaged areas to a temperature at which the prior image is erased.

The formation of the electrostatic latent image in these prior systems is often difiicult. Often, it is necessary to coat the deformable material over a photoconductive layer or mix a photoconductor in the deformable material so that the material can be uniformly electrostatically charged, then exposed to a light image to cause the charge to dissipate only in background areas. The inclusion of a photoconductive material in the deformable material or as a sublayer adds a serious complication to the system. Attempts have been made to form the electrostatic latent image by electron beam bombardment. However, since the electron beam bombardment must take place in a vacuum, it is often difficult to heat the layer to permit the surface deformation to take place since the heating must either occur in the vacuum chamber immediately after image formation or must take place after release of the vacuum and removal of the material from the vacuum system. Any excessive delay between the image formation and the heat development step is likely to permit the charge pattern to dissipate.

Not all thermoplastic materials are suitable for use in these known surface deformation imaging systems. In general, the materials which give the best surface deformation images are low molecular weight, low melting point thermoplastics. These materials tend to have tacky surfaces which are easily damaged in handling and are easily contaminated, such as by airborne dust particles. After the surface deformation image is produced, fixing the image against accidental erasure by later heating is difficult. Chemical treatment or overcoating with other materials is often necessary if a permanent image is required.

Another imaging technique utilizing a polymeric material is known as photopolymerization imaging. -In such a system, a layer is first formed comprising a monomer or low molecular weight polymer which is capable of further polymerization or cross-linking upon exposure to suitable radiation. Such materials are widely used to form resists for etching processes. In such a process, a surface is coated with the photopolymer, the coating is exposed to an image by ultra violet light, and which causes the exposed areas to polymerize. Then the unpolymerized background areas are removed by a solvent which does not attack the image photopolymer or in which the differential solubility between the polymerized and unpolymerized areas is suflicient to permit removal of the unpolymerized material without destroying the polymerized areas. The substrate in the background areas may then be treated or dissolved away by a suitable solvent.

While photopolymers are highly useful in such etching processes, they have not been found to be commercially useful for visible image formation. Immediately after exposure, the images are not visible. Further treatment, such as removal of the background areas, is necessary to give a detectable image. In order to make the image visible to the naked eye, a dye must be included in the photopolymer or the polymerized image must be treated to make it visible.

Thus, it can be seen that while the present polymeric surface deformation imaging systems have several useful characteristics, problems remain which prevent their widespread use, especially in electron beam recording areas.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a surface deformation imaging system which overcomes the above-noted problems.

It is another object of this invention to provide an electron beam recording system which produces an instantaneously visible surface deformation image.

It is another object of this invention to provide a surface deformation imaging system which does not require heating the imaging material.

It is another object of this invention to provide a surface deformation imaging system which produces a fixed image without treatment of the material after exposure.

Still another object of the invention is to provide a surface deformation imaging system which permits selected areas of the imaging materials to be exposed and fixed at different times.

The above objects, and others, are accomplished in accordance with this invention by providing a surface deformation electron beam imaging system which includes the steps of forming a layer of a highly viscous photopolymer, exposing the photopolymer to an electron beam in a vacuum, whereby the exposed areas on the photopolymer are simultaneously polymerized and form a surface deformation image. The photopolymer layer comprises a material which is capable of polymerization or cross-linking when subjected to electron beam bombardment. The material could be a monomer, a mixture of monomer and polymer, or a prepolymer which further polymerizes or cross-links when irradiated. For simplicity, the term photopolymer is used throughout this application to include materials having these functional characteristics.

Background areas on the photopolymer layer may be fixed by exposure to suitable radiation, such as ultra violet light, which completes the polymerization process throughout. Thus, different areas of the layer may be imaged by the electron beams at different times followed by eventual exposure of the entire surface to ultra violet light to fix the material between imaged areas. No heating or chemical treatment of the layer is required.

While the mechanism by Which this imaging process operates is not fully understood, it is thought that it is a combination of photopolymerization and electrostatic charge repulsion processes operating simultaneously in those areas bombarded by the electron beam. The polymerization process proceeds through the formation of free radical monomers in those area penetrated by the electron beam. The surface deformation appears to be due to changes in the viscosity of the layer in a rand-om manner depending on such things as charge distribution on the layer surface. During the polymerization in the exposed areas, the highly viscous material is capable of migration into the polymerizing areas. The non-uniform reaction results in microscopic cracks in the layer at the surface giving a cracked, folded or wrinkled appearance which scatters light. The effect of differing voltages on the surface deformation image appears to be related to the penetration of the electron beam into the photopolymerizable layer. At low voltages only the surface, or a portion of the surface, polymerizes. As the beam voltage increases, the layer is penetrated to greater depth, resulting in more uniform polymerization. Since the surface is then less able to flow and deform in response to electrostatic repulsion forces caused by the electrostatic charge pattern created by the electron beam, less surface deformation and smaller cracks occur. Ultimately, at a sutficiently high voltage, a completely homogeneous reaction occurs and the polymer forms with no light scattering surface deformation.

This system has high photographic speed resulting from the fact that one electron can form many radicals. The polymerization process proceeds by a free radical mechanism causing many monomers to react for each radical initially formed. It appears that the characteristics of the surface deformation image are both voltage and current sensitive. It has been found that as the current is increased (with constant voltage) the intensity of the surface deformation image also increases. Also, as the voltage is increased at constant current, the coarseness of the surface deformation decreases until at sufiiciently high voltages no surface deformation occurs. It has been found that a useful combination of photopolymerization and surface deformation imaging occurs over a voltage range of from about 5,000 to about 23,000 volts. The surface can be polymerized without surface deformation at voltages above about 30,000 volts. This is useful, for example, to permit fixing of backround areas. The useful current density range has been found to be from about 10 amp/cm. to about 1'0 amp./cm. Depending upon the image characteristics required, it may be desirable to use low Voltages with high currents or high voltages with low currents.

The exposure to the electron beam should take place in a vacuum. In general, a vacuum of 10- torr or better is preferred. Excessive gas in the vacuum chamber decreases the effectiveness of the electron beam and causes scattering of the beam which decreases image resolution. Further, it is very desirable that oxygen be excluded during this step since oxygen tends to inhibit the polymerization reaction.

Any conventional electron beam generating system may be used. For example, the imaging material may be scanned in a raster made and the beam current modulated by a conventional television video signal to produce alphanumeric or pictorial images. Alphanumeric information can also be imaged by techniques in which dots or short lines are produced on the imaging material by the electron beam in patterns which form the desired characters. An especially useful character generating technique (such as is described in US. Pat. No. 2,735,956) utilizes a character-shaped electron beam.

Any suitable photopolymerizable material may be used in the photo-sensitive layer. Results are often improved by the addition of polymerization initiators and thermal polymerization inhibiters. Best results are generally obtained with photosensitive layers comprising from about 10 to about weight percent of an addition polymerizable ethylenically unsaturated compound and from about 90 to about 10 Weight percent of a thermoplastic polymer. Often, image quality and process efiiciency can be further improved by the incorporation of up to about 10 weight percent of an addition polymerization initiator and up to about 3 weight percent of a thermal polymerization inhibitor.

Typical thermoplastic polymers include polyamides such as N-methoxymethyl polyhexamethylene adipamide; vinylidene chloride copolymers, such as vinylidene chloride/acrylonitrile and vinylidene chloride/methacrylate copolymers; polyurethanes; polycarbonates; polystyrenes; polyolefins; cellulose esters, such as cellulose acetate and cellulose acetate butyrate; polyvinyl esters such as polyvinyl acetate; polyacrylates such as polymethyl methacrylate; polyethylene oxides; polyalkylene glycols; polyvinyl acetals such as polyvinyl butyral; and mixtures and copolymers thereof.

Typical addition polymerizable ethylenically unsaturated esters of polyols such as ethylene diacrylate; glycerol diacrylate and 1,3-propanediol dimethylacrylate; the bis-acrylates and methacrylates of polyethylene glycols; unsaturated amides such as methylene bis-acrylamide, ethylene bis-methacrylamide; diethylene triamine trismethacrylamide and 1,6-hexamethylene bis-acrylamide; vinyl esters such as divinyl succinate and divinyl phthalate; unsaturated aldehydes such as sorbaldehyde; and mixtures thereof. e

Typical addition polymerization initiators include substituted and unsubstituted polynuclear quinones such as 9,10 anthraquinone, 9,-l0-phenanthraquinone, 2-phenylanthraquinone and 1,2,3,4-tetrahydr0benz(a) anthracene- 7 ,12-dione; vicinal ketaldonyl compounds such as diacetyl benzil; ketaldonyl alcohols such as benzoin and pivaloin; acyloin others such as benzoin methyl ethers and mixtures thereof.

Typical thermal polymerization inhibitors include pmethoxyphenol, hydroquinone, terbutylcatechol, naphthylamines, pyridine, chloranil, and mixtures thereof.

Other compounds may be added to the photosensitive layer in order to improve photosen'sitivity, adhesion to the substrate, chemical inertness, etc., if desired. Typically, non-thermoplastic polymers such as polyvinyl alcohol, cellulose or gelatin may be added to improve the surface and wear characteristics of the layer, if desired.

In general, best results are obtained where the photosensitive layer contains precursors producing a methacrylate polymer or methacrylate copolymer. It is also preferred that polyethylene glycol diacrylate be used where it is desired that the layer include a thermoplastic polymer in addition to the photopolymerizable material. Where a polymerization initiator is used, 9,10-anthraquinone is preferred. If a thermal inhibitor is desired, p-methoxyphenol gives preferred results. Images of optimum quality and stability are produced when the above noted preferred ingredients are used in combination.

Since the image in those areas contacted by the electron beam is substantially fixed due to the combined surface deformation and photopolymerization in those areas, it is possible to image only selected areas on a larger photopolymer layer surface. Later, the layer may be reinserted in the vacuum chamber and images formed on other portions of the sheet. Since the exposed area may not be entirely fixed, e.g. clue to unexposed areas between image characters, it is generally desirable that a discrete portion of the surface including the exposed area be fixed, such as by ultraviolet irradiation immediately after exposure. This fixing step may not be necessary where images are to be formed on'the remaining portions within a short time. When all of these desired areas have been imaged, the overall sheet may be fixed as described above.

It is preferred that the photopolymer be coated on a supporting substrate since many photopolymers are relatively weak and may not be self-supporting. Any suitable substrate may be used. Typical substrates could include plastic film such as polyethylene terphthalate and polyvinylidene chloride, glass, metal, paper, etc. A photopolymer layer of suitable thickness, after complete polymerization, may be self-supporting. In such an instance, it is possible to strip the polymerized image layer from the support after the above-describing fixing. Where the layer is to be stripped from the support, the support should have a surface which adheres poorly to the photopolymer layer. Typical strippable supports include fluorocarbon materials, polished metals such as stainless steel, which may be Waxed or silicone coated, so long as the coating does not interfere with proper polymerization of the photopolymerizable layer.

Preferably, the photopolymerizable layer has a thickness of from about 0.2 to about 1.5 mils. Much thinner layers produce poor surface deformation images, while much thicker layers are diflicult to polymerize quickly and completely.

Since the photopolymer layer remains tacky in background areas after electron beam image formation, a solid body, such as a transparent film, may be pressed against the layer and will adhere thereto. The layer may then be uniformly polymerized, such as by ultra violet irradiation through a cover film. If desired, the layer may be stripped from a substrate before or after this uniform polymerization step. This laminating technique has the advantages of providing support for the layer and of protecting the surface deformation image from abrasion damage, since the image is at the layer-film interface.

If the adhered solid body is a transparent film, the image may be viewed by projection, with the light-scattering surface deformation image appearing dark against a clear background. Conversely, if the solid body is opaque, e.g. a sheet of black paper, the image may be viewed by reflection, with the light-scattering surface deformation image appearing white against a dark background.

6 'DETAIIJED DESCRIPTION 01F THE INVENTION The following examples further describe the invention in detail and point out several preferred embodiments thereof. Parts and percentages are by Weight unless otherwise indicated.

EXAMPLE I The imaging material is prepared by mixing together about parts polyethylene glycol diacrylate, about 1 part phenanthrenequinone and about 100 parts cellulose acetate butyrate in sufficient acetone to give a solution containing about 10 percent solids. This solution is coated onto a polyethylene terephthalate substrate to a wet thickness of about 1.5 mils. After drying for 2 hours at a temperature of about 50 C., the layer has a thickness of about 0.5 mil and is tacky to the touch.

The resulting composite sheet is placed on a grounded aluminum plate in a demountable cathode ray tube which is then closed and evacuated, to a vacuum below about 10- torr. An electron beam, operated at a voltage of about 10 kv. and a current density of about 10* amps/cm. is caused to scan the sheet in a conventional manner to produce an image. As the electron beam ex posure proceeds, the exposed areas are seen to form microscopic cracks or wrinkles, giving these areas a frosted appearance. The sheet is then removed from the tube and exposed to ultra violet radiation from a watt short are mercury lamp for about 1 minute at a. lamp-to-sheet distance of about 1 8 inches in a nitrogen atmosphere. The sheet is found to be no longer tacky and to have an excellent light scattering image on a clear, handleable background.

EXAMPLE II A solution is prepared by mixing together about 1200 parts low viscosity polyvinyl acetate methacrylate, about 250 parts polyethylene glycol diacrylate, about 1 part anthraquinone and about 1000 parts ethyl alcohol. The solution is coated onto an aluminum sheet by means of a wire wound rod. The coating is dried for about 1 hour at about 55 C., resulting in a tacky layer having a thickness of about 0.4 mil. The coated sheet is placed in a container containing both an electron beam source and a source of ultra violet radiation. The aluminum substrate is grounded. The container is closed and evacuated to a vacuum of about 10- torr. The electron beam, operated at a voltage of about 15 kv. and a current density of about 10* amps/cm. is caused to scan the sheet in a conventional video manner to produce an image. The areas struck by the electron beam are seen to develop a frosted appearance. The ultra violet source is then activated, irradiating the sheet for about 2 minutes. The container is opened and the sheet is removed. An excellent, high resolution light scattering image results, on a fixed, non-tacky background.

EXA MPDE III A solution is prepared by mixing together about 3000 parts of an aqueous solution of polyethylene oxide (having a molecular weight of about 100,000), about parts polyethylene glycol diacrylate, about 0.4 part anthraquinone and about 0.2 part p-methoxyphenol (as a thermal inhibitor). The solution is coated onto a polished stainless steel plate to a wet thickness of about 10 mils. After drying at about 50 C. for about 2 hours, the layer has a thickness of about 1 mil. The plate is then placed in a demountable cathode ray tube of the shaped beam type (as described, for example, in US. Pat. No. 3,111,598). The tube is evacuated to a vacuum of about 10 torr. The plate is connected to a potential source and maintained at a positive potential of about 200 volts. The tube is then operated to expose the layer to an image comprising a plurality of alphanumeric characters. The shaped electron beam is operated at a negative potential of about 15 kv. and a current density of about 10 amps/cm After exposure, the characters are seen on the layer as frosted areas. The plate is removed from the tube and a 5 mil polyvinylidene chloride film is placed over the film, in contact therewith. Since unexposed areas are tacky, the film bonds well to the layer. The layer is then exposed to ultra violet radiation from a 200 watt short arc mercury lamp for about 3 minutes at a lamp-to-film distance of about inches. The layer is then carefully stripped from the stainless steel plate. The resulting film is suitable for projection, with the light-scattering character image areas appearing black against a clear background when projected. Since the images are at the film-layer interface, they are protected against abrasion damage when the composite is handled.

EXAMPLE IV A solution is prepared by mixing together about 350 parts cellulose acetate butyrate (having a viscosity of about 100 poise), about 7 parts phenanthrenequinone, about 350 parts polyethylene glycol diacrylate (having an average molecular weight of about 300) and about 0.35 part p-methoxyphenol in about 3000 parts acetone. The solution is coated onto a polyester film base and dried to a thickness of about 0.4 mil. The composite sheet is placed on a grounded metal plate in a demountable cathode ray tube, which is then evacuated to a vacuum of about 10- torr. The sheet is then imaged by operating the electron beam in a conventional video mode. The electron beam is operated at a voltage of about 12. kv. and a current density of about 10- amps./cm. After imaging is completed, the exposed areas have a frosted appearance. The entire layer is then scanned by the electron beam at a voltage of about 50 kv. At this potential, surface deformation does not occur, but the layer is polymerized. The tube is then opened and the sheet is removed. An excellent image on a fixed, non-tacky, clear background results.

The materials and physical variables used in the above examples illustrate specific preferred embodiments of the present invention. Other suitable materials and process steps, as described above, may be used with similar results. In addition, other materials may be added to the photopolymer layers to synergize, enhance, or otherwise modify their properties. For example, dye sensitizers may be used if desired.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

1 claim:

1. A surface deformation imaging process comprising the steps of providing a tacky-surfaced layer on a support, said layer comprising a photopolymerizable ingredient; exposing at least a portion of the surface of said layer to an electron beam in image configuration at a voltage of from about 5,000 to about 23,000 volts and a current density of from about 10- to about 10* amperes per square centimeter; whereby a surface deformation image is formed and said ingredient is polymerized in exposed areas; and pressing a solid body against said layer so that the body bonds to the layer in tacky unexposed areas.

2. The process according to claim 1 further including the steps of substantially uniformly polymerizing the unexposed areas of said layer without further surface de formation, then stripping said layer from said support.

3. The process according to claim 2 wherein said body is substantially transparent to ultraviolet radiation, and said uniform polymerization is accomplished by exposing said layer to ultraviolet radiation through said body.

4. The process according to claim 1 further including the steps of stripping said layer from said support, then substantially uniformly polymerizing the unexposed areas of said layer without further surface deformation.

5. The process according to claim 1 wherein said layer comprises from about 10 to about weight percent of an addition polymerizable ethylenically unsaturated compound as the photopolymerizable ingredient and from about 90 to about 10 weight percent of a thermoplastic polymer.

6. The process according to claim 1 wherein the photo polymer produced is a composition selected from the group consisting of methacrylate polymers, copolymers and mixtures thereof.

7. A surface deformation imaging process comprising the steps of (a) providing a layer comprising a photopolymerizable ingredient;

(b) exposing a first portion of the surface of said layer to an electron beam, in image configuration, at a voltage of from about 5,000 to about 23,000 and a current density of from about 10- to about 10* amperes per square centimeter, whereby a surface deformation image is formed and said ingredient is at least partially simultaneously polymerized in exposed areas;

(0) substantially uniformly polymerizing the layer without further surface deformation over a discrete area including said first portion;

(d) repeating steps (a) through (c) with at least one additional discrete area of said layer surface; and

(e) substantially uniformly polymerizing without further surface deformation any remaining unpolymerized areas on said layer.

8. The process according to claim 7 wherein said layer has a tacky-surface prior to uniform polymerization, and a solid body is pressed against said layer prior to uniform polymerization, causing said body to adhere to remaining tacky unexposed areas of the layer surface.

9. The process according to claim 8 wherein said body is substantially transparent to ultraviolet radiation, and said uniform polymerization is accomplished by exposing said layer to ultraviolet radiation through said body.

10. The process according to claim 7 wherein said layer comprises from about 10 to about 90 weight percent of an addition polymerizable ethylenically unsaturated compound as the photopolymerizable ingredient and from about 90 to about 10 weight percent of a thermoplastic polymer.

11. The process according to claim 7 wherein the photopolymer produced is a composition selected from the group consisting of methacrylate polymers, copolymers and mixtures thereof.

12. A surface deformation imaging process comprising the steps of providing a layer having a thickness of from about 0.2 to about 1.5 mils comprising from about 10 to about 90 weight percent of a thermoplastic polymer, from about 90 to about 10 weight percent of an addition polymerizable ethylinically unsaturated compound and up to about 10 weight percent an addition polymerization initiator; exposing said layer in image configuration to an electron beam at a voltage of from about 5,000 to about 23,000 volts and a current density of from about 10" to about 10- amperes per square centimeter in a vacuum of less than about 10" torr; whereby simultaneously a surface deformation image is formed and said polymerizable compound is at least partially polymerized in areas exposed to said electron beam; then polymerizing the remainder of said polymerizable compound across said layer in the substantial absence of oxygen without further surface deformation.

13. The process according to claim 12 wherein the photopolymcr produced is a composition selected from the group consisting of methacrylate polymers, copolymers and mixtures thereof.

14. The process according to claim 13 wherein said layer is coated on a support, and said process includes the further step of stripping said layer from said sub- 3,348,944 10/1967 Michalchik 9635.l X strate after completion of uniform polymerization. 3,530,100 9/1970 DAlelio 204l59.16 X

References Cited CARL D. QUARFORTH, Primary Examiner 5 E. Assistant Examiner 3,297,440 1,1967 ifi 9645i 96-35.1, 46; 206-15916; 25065 R; 26 -22 3,330,659 7/1967 Wainer 9635.1 10 

